Mixture control unit for use with the control system of a soil venting apparatus

ABSTRACT

The invention provides a mixture control system for controlling the flow of vaporized hydrocarbons drawn into an engine for combustion and combining a supplemental fuel with the vaporized hydrocarbons as required to maintain a near stoichiometric mixture.

This is a division of application Ser. No. 07/512,305 filed Apr. 20,1990 now U.S. Pat. No. 5,070,850, issued Dec. 10, 1991.

FIELD OF THE INVENTION

The present invention relates generally to systems for venting andcombustion of hydrocarbon vapors present in soil, land fills, and emptystorage tanks. More particularly, the invention provides a mixturecontrol system for controlling the flow of vaporized hydrocarbons drawninto an engine for combustion and combining a supplemental fuel with thevaporized hydrocarbons as required to maintain a near stoichiometricmixture.

BACKGROUND OF THE INVENTION

Spills of hydrocarbons into soil, hydrocarbon gases generated bymaterials in land fills, and residual vapors in empty storage tanks mustbe extracted and disposed of to prevent environmental impact through airpollution, toxic hazard, or explosion hazard. Typical systems forachieving this end employ vacuum pumps to create a pressure gradientthrough perforated pipe embedded in the contaminated soil or land fillmaterial, or lines running to an empty storage tank. The vaporizedhydrocarbons are drawn through the vacuum pump into a combustion unitwhere they are eliminated by burning.

An improved system disclosed in U.S. Pat. No. 4,846,134 employs aninternal combustion engine which provides vacuum to the system throughthe intake manifold of the engine. The extracted waste hydrocarbonsprovide all or some of the fuel for the engine and are destroyed bycombustion in the cylinders. This system provides unique advantages inthat supplemental power requirements are minimized for operation of thesystem.

The control system for starting the engine on a supplemental fuel andmixing the extracted vapor with the supplemental fuel for combustion haspreviously employed manual control. Valves connecting the supplementalfuel and the hydrocarbon vapor source to the engine have been controlledby a human operator to provide optimum mixture of the fuel, comprisingthe supplemental fuel and hydrocarbon vapors, and air drawn both fromthe hydrocarbon vapor source and the atmosphere.

Such operation requires trained operators and full time monitoring ofthe system to provide optimum efficiency. For cost reduction andefficiency improvement, it is desirable to provide a control system forthe vapor extraction system which is automated and provides a propermixture to the engine for most efficient destruction of the hydrocarbonsand minimum pollution from the exhaust gases of the engine.

SUMMARY OF THE INVENTION

The present invention provides a control system for an internalcombustion engine based vapor extraction and combustion system. Theinvention employs an engine having a standard gaseous fuel carburetorfor initial control of the engine on a supplemental fuel. A mixingchamber is provided to receive the supplemental fuel, vaporized wastehydrocarbons as well as entrained air from a hydrocarbon source, and airreceived through the carburetor, to ensure appropriate mixing of thecharge entering the manifold of the engine. The vaporized hydrocarbonsand entrained air, or source gas, from the hydrocarbon source flowthrough a source gas shutoff valve. The source gas flows from the sourcevalve to a mixture control unit which also is connected to asupplemental fuel tank through a three-way valve which feeds thesupplemental fuel from the tank to the gas carburetor or to the mixturecontrol unit. An oxygen sensing system detects oxygen concentration inthe exhaust manifold of the engine and a control means responsive to theoxygen concentration adjusts the mixture control unit to supply fuel tothe engine from the source gas richened with the supplemental fuel ifnecessary to provide a near stoichiometric mixture to the engine. Acontrollable bypass valve is provided from the source gas shutoff valveto the mixing chamber allowing a portion of the source gas to bypass themixture control unit when the heating value of the source gas becomeslow enough that a sufficient volume of gas cannot be provided throughthe mixture control unit due to flow restrictions within the unit.Control of the bypass valve is responsive to the supplemental fuel flowrate.

Engine speed is controlled by a butterfly valve in the gaseous fuelcarburetor controlling air flow from the atmosphere through thecarburetor to the intake manifold. A control unit responsive to enginespeed adjusts the butterfly valve position thereby governing speed ofthe engine. Since air may be entrained in the source gas, thecontrollable bypass valve is also responsive to the speed control unitin a condition when the butterfly valve is closed due to the entrainedair in the source gas providing sufficient combustion oxygen through thebypass valve.

Integrated control of the mixture control unit, bypass valve, butterflyvalve, and in certain embodiments, the three-way valve and sourceshutoff valve is accomplished through the use of a microprocessor.

Operation of the system is initiated by positioning the three-way valveallowing supplemental fuel from the tank to feed through the gaseousfuel carburetor to the mixing chamber and intake of the engine forstarting. After smooth operation of the engine is achieved, thethree-way valve is repositioned to divert the flow of supplemental fuelthrough the mixture control unit into the mixing chamber. Exhaust oxygenconcentration is monitored to provide control input for a controller forthe mixing control unit which adjusts the supplemental fuel flow toachieve near stoichiometric operating conditions. The source shutoffvalve is then opened allowing source gas to enter the mixture controlunit. The hydrocarbons in the source gas richen the mixture providedthrough the mixture control unit which is sensed in the exhaust gasoxygen concentration. The mixture control unit reduces the supplementalfuel flow to maintain a near stoichiometric mixture. If the heatingvalue of the source is sufficiently high, the mixture control unit willcompletely eliminate supplemental fuel flow to the engine. Engine speedis controlled through the butterfly valve which is positioned inresponse to the engine speed.

As the hydrocarbon mixture in the source gas is reduced in heating valueproviding a leaner mixture to the engine, the mixture control unit willagain begin feeding supplemental fuel to the engine. Flow of thesupplemental fuel is sensed and control of the bypass valve initiated toproduce additional flow of source gas bypassing the mixture control unitand entering the mixing chamber directly. The mixture control unit thenprovides mixture control by controlling a portion of the source gas flowand the supplemental fuel flow. Continued flow of supplemental fuelresults in further opening of the bypass valve until fully open. Enginespeed control is maintained through the butterfly valve until sufficiententrained air is provided in the source gas to meet oxygen needs forcombustion. When the engine speed governor has closed the butterflyentirely, engine speed control is maintained by controlling the sourcegas flow through the bypass valve. Mixture control is maintained by themixture control unit metering supplemental fuel and a portion of thesource gas to maintain near stoichiometric mixture in the engine. Whenthe hydrocarbons in the source gas are sufficiently depleted to requireoperation on supplemental fuel only, the initial operating sequence ofthe engine may be conducted in reverse to achieve a smooth shut down ofthe system.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the invention may be better understoodwith reference to the following drawings, detailed description and theappended claims.

FIG. 1 is a schematic of the engine supplemental fuel supply, sourcesupply, and the mixture control system;

FIG. 2 is a side view cutaway of the mixture control unit;

FIG. 3a is a top view of the mixing chamber;

FIG. 3b is a right side view of the mixing chamber; and,

FIG. 3c is a front view of the mixing chamber.

DETAILED DESCRIPTION OF THE INVENTION

A system employing an internal combustion engine and plumbing system forvolatilizing hydrocarbon vapors and combusting them for pollutionreduction is disclosed in U.S. Pat. No. 4,846,134 dated July 11, 1989,having a common assignee with the present invention, the disclosure ofwhich is fully incorporated herein by reference. Referring now to thedrawings, FIG. 1 shows a schematic representation of an exampleembodiment of the present invention. An internal combustion engine 10 isused to provide manifold vacuum for volatilizing and aspiratinghydrocarbons from a source such as contaminated soil, land fill mass, oran empty storage tank. A generalized source for this gas is designated12 in FIG. 1. The engine includes a intake manifold 14 and an exhaustmanifold 16, a catalytic convertor and muffler system 18 is attached tothe exhaust manifold for reducing emissions from the engine exhaust. Acarburetor 20 and mixing chamber 22 are connected to the intake manifoldfor fuel supply to the engine as will be described in greater detailsubsequently. Primary combustion air enters the engine through thecarburetor via air cleaner 24.

A supplemental fuel is provided for initial operation of the engine andsupplementing the combustible hydrocarbons in the source gas asrequired. In the embodiment shown in FIG. 1, an LPG supply 26 isemployed. Gaseous fuels are preferred for use as the supplemental fuelsource and may include LPG, propane, methane, compressed natural gas orliquified natural gas. The carburetor is adapted for gaseous fuel use. Aflow meter 28 measures fuel flow from the supplemental supply, as willbe described in greater detail subsequently. For LPG or other liquifiedgaseous fuels, a convertor regulator 30 is provided to vaporize andtemperature stabilize the fuel as known to those skilled in the art. Thevaporized supplemental fuel flowing in the supply line 32 is providedthrough a three-way valve 34 to a carburetor fuel line 36. With thethree-way valve in a first position, supplemental fuel flows through thecarburetor fuel line entering the carburetor for mixing with primarycombustion air then flowing through the mixing chamber into the intakemanifold of the engine. Primary air flow through the carburetor iscontrolled using a butterfly valve 37. Since the engine operates underno-load conditions, a speed sensor is employed to avoid engineoverspeed. An engine speed controller 38 positions the butterfly valveto maintain engine speed for given fuel flow. With the three-way valvepositioned in a second position, supplemental fuel flows throughsupplemental fuel line 39 to a mixture control unit 40 (MCU). The sourcegas from the contaminated soil, well, tank, or land fill comprising thesource 12 is provided through a source gas shutoff valve 42 to a sourcegas control line 44 connected to the MCU. The MCU then provides thesource gas, supplemental fuel or a combination of the two, as will bedescribed in greater detail subsequently, to the mixing chamber throughmixture control fuel line 46.

The fuel mixture provided to the engine through the mixing chamber iscontrolled in the MCU. A mixture sensing means attached to the engineproduces a control signal which enables the MCU to adjust the flow ofsource gas and supplemental fuel as required to provide sufficient fuelto create a near stoichiometric mixture in the mixing chamber. In theembodiment shown in FIG. 1, an oxygen sensor 48 provides the mixturemeasurement. A controller such as a microprocessor 50 receives the inputfrom the oxygen sensor and provides an output to control the MCU. Themicroprocessor may be a single integrated unit or a plurality ofinterconnected units governing separate functions in the overall system.In the embodiment shown, a dithering valve 52 receives the output fromthe controller to adjust a vacuum level provided to the MCU throughvacuum line 54. As will be described in greater detail subsequently, thevacuum level provided to the MCU controls the mixture of the source gasand supplemental fuel supplied through mixture control fuel line 46.

In the embodiment shown in the drawings, the components described arecommercially available. The microprocessor is a combination of twounits; a first analog processor receiving oxygen level information fromthe oxygen sensor and providing control output to the dithering valveand a process controller receiving data from and controlling otherfunctions to be described subsequently. The dithering valve is availablefrom Borg Warner, the analog processor is produced by Autotronics Corp.under part number 4045 TMS, the process controller is a Kaye X1510Process Link with a V222 Analog Scanner, and the oxygen sensor isavailable from Autotronics Corp. under part number 8945.

Flow of source gas through the MCU is limited. As the heating value ofthe source gas reduces during depletion of gas from the source,sufficient volume of source gas cannot be passed through the MCU tomaintain a near stoichiometric mixture without the use of supplementalfuel. However, if provided in greater volume, sufficient heating valuein the source gas exists to operate the engine with minimum use ofsupplemental fuel, a bypass valve 56 is provided to route source gasthrough bypass line 58 directly to the mixing chamber.

The volume of source gas provided through bypass line 58 to the mixingchamber includes entrained air from the source. Typically, the sourcegas is provided from a contaminated soil, a fuel tank undergoinginerting, or a land fill, each of which contains significant amounts ofoxygen or allows air to be drawn through the system. Consequently, asbypass valve 56 is opened and source gas is provided to the mixingchamber, a reduction in primary air will be needed to maintain enginespeed. The engine speed sensor and associated control for the carburetorbutterfly valve performs this function until the butterfly valve hasbeen completely closed. When the butterfly valve has been closed, thebypass valve is controlled to maintain engine speed.

In the embodiment shown in FIG. 1, the bypass valve is controlled by themicroprocessor and may employ a standard solenoid operated valve orother electrically controllable valve as known to those skilled in theart. The electrical bypass control line 60 is shown in FIG. 1. Themicroprocessor initiates opening the bypass valve during operation ofthe engine when the MCU begins adding supplemental fuel to racine themixture. The microprocessor senses supplemental fuel flow through flowmeter 28 on control line 62. As will be described in greater detailsubsequently, operation of the engine requires starting on thesupplemental fuel. Consequently, to avoid opening of the bypass valveprematurely, a source gas flow switch 64 is provided to sense completeopening of the source shutoff valve 42. The source gas flow switch shownin the embodiment in FIG. 1 is a separate manually operated switchconnected to the microprocessor through control line 66 which is engagedupon complete opening of the source gas shutoff valve by the operator.An automatic contacting switch sensing the fully open position of thesource gas shutoff valve or a flow rate sensor for the source gas couldbe employed as replacements for the manual source gas flow switch. Themicroprocessor inhibits control of the bypass valve unless the sourcegas flow switch is engaged.

A general purpose computer 70 is interfaced to the microprocessor in theembodiment shown in FIG. 1 to provide data recording and externalcommunication capability for the system. A standard personal computeravailable from numerous sources may be employed with appropriatecommunications interfaces.

Referring now to FIG. 2, a detailed drawing of the mixture control unitof a present embodiment is provided. The mixture control unit includes abody 210 which in the embodiment shown comprises an upper casting 210Aand a lower casting 210B. The mixing unit receives the source gas fromsource control line 44 of FIG. 1 through aperture 212. Supplemental fuelis received by the mixture control unit from supplemental fuel line 39of FIG. 1 through a second aperture 214. The fuel mixture of source gasand supplemental fuel flows from the mixing control unit into mixturecontrol fuel line 46 through a third aperture 216. In the embodimentshown, each of the apertures comprises an internally threaded nipple forreceiving a threaded fitting attaching the various fuel lines. Thesource gas flows through a first valve 218 having a seat 220 surroundingthe aperture and engaging a valve cap 222. The valve cap is mounted toan arm 224 which is in turn pivotally supported through pin 226 allowingthe valve cap to be rotated off the valve seat. A lever 228 extendingfrom the arm opposite the pivot pin 226 engages a first spring 230urging the lever upwards to rotate the arm downwards about pin 226 toseat valve cap 222. A plunger 232 extending through an orifice 234 inthe upper casting engages the lever urging it downwards through reactionof a bonnet spring 236. A bonnet casting 238 constrains the bonnetspring and forms a vacuum chamber in combination with the upper bodycasting. A diaphragm 240 separates the vacuum chamber into an upperchamber 242 and a lower chamber 244. A washer 246 connects the spring,plunger and diaphragm. A first air bleed 248 maintains atmosphericpressure in the lower chamber. Vacuum is applied through a vacuum port250 extending through the bonnet into the upper chamber. A bleed airport 252 provides equalizing pressure for the upper chamber allowing thediaphragm to reposition upon removing vacuum from the vacuum port. Thediaphragm moves through a range from a first position with maximumvacuum on the upper chamber through an intermediate position,essentially undeformed horizontally, to a second position with minimumvacuum on the upper chamber.

With no vacuum present, the bonnet spring 236 forces the diaphragm andwasher downwardly, moving the plunger through the orifice in the uppercasting to depress the lever 228 of the first valve assembly. A secondvalve assembly 254 is provided for the supplemental fuel input aperturehaving similar components to the first valve including a valve seat,256, valve cap 260, second arm 262, second pivot pin 264 and secondlever 266. A second spring 268 engages the second lever urging thesecond lever upwardly, rotating the second arm about the second pivotpin to close the second valve cap on the second valve seat. The secondlever is configured as shown in FIG. 2 with a downward angle placing thesecond lever below the first lever to be engaged by the plunger afterthe first valve is completely open. With the second spring 236 in thefully extended position driving the diaphragm, washer, and plunger tothe second or downward most position, the first and second valves areboth fully open. The application of maximum vacuum to the upper chambercauses the diaphragm to deform to the first or upward most position.Intermediate vacuum levels placing the diaphragm in a range between thefirst position and intermediate position, position the plunger tocontrol the first valve between a fully closed and fully open position.Intermediate vacuum levels placing the diaphragm in a range between theintermediate position and the second position, position the plunger tocontrol the second valve between a fully closed and fully open position.Incremental changes in vacuum between the maximum and minimum levelsallows smooth operation of both valves through a continuous range.Source gas flowing through the first valve and supplemental fuel flowingthrough the second valve intermix in the central chamber 270 of the MCUbody flowing to the main fuel line through aperture 216.

As shown in FIG. 2, for an embodiment employing LPG as the supplementalfuel the diameter of the aperture and valve for the source gas exceedsthe diameter of the aperture and valve for the supplemental fuel. Largerdiameters in the source gas valve are required to accommodate therelatively lower heating value of the source gas with respect to thesupplemental fuel.

An air mixing bleed port 272 is provided into the central chamber in thebody of the MCU to accelerate gas mixing and transport from the MCU tothe mixing chamber by increased volume flow. Reducing diameter andlength of the main fuel line from the MCU to the mixing chamber reduceslag time in transmission of fuel from the MCU to the engine to avoidover-controlling or oscillatory instability.

Vacuum is provided to the MCU from the manifold of the engine through adithering vacuum valve 52 as described with respect to FIG. 1. In theembodiment shown, the dithering valve employs pulse code modulation(PCM) to provide a relative duty cycle and consequent vacuum level inthe upper chamber of the bonnet proportional to the rich or leancomposition of the fuel air mixture as determined by the oxygen sensorpresent in the exhaust manifold of the engine. As previously described,a microprocessor receiving data from the oxygen sensor representative ofthe overall mixture level provides output to the dithering valve whichin turn provides control for the MCU.

As previously described, the engine is started using supplemental fuelwith the three-way valve providing the gaseous supplemental fueldirectly to the standard gaseous carburetor on the engine. When theengine is warm and the oxygen sensor is hot enough to operate normally,the three-way valve is repositioned to provide the supplemental fuelthrough the MCU. Control of the three way valve may be manual or undercontrol of the microprocessor. A temperature sensor or other means toverify sufficient engine warm-up for operation of the oxygen sensor isrequired for input to the microprocessor prior to actuating thethree-way valve. During startup and warming of the engine, the standardgaseous carburetor provides the appropriate fuel mixture to the engine.A lean mixture is supplied by the carburetor to provide an optimumoperating point for initiation of MCU control. Once the oxygen sensorbegins providing control output to the microprocessor, the ditheringvalve begins operation under the command of the microprocessor supplyingvacuum to the MCU.

Supplemental fuel flow is transferred from the gaseous carburetor to theMCU by operation of the three-way valve. Prior to operation of thevalve, sensing of a lean mixture results in commands by themicroprocessor to decrease the vacuum applied to the diaphragm allowingair entering the bleed port 252 to force the diaphragm downwardly,positioning the plunger to open both valves in the MCU. As supplementalfuel is introduced into the MCU, a rich mixture is sensed and increasedvacuum will be applied to the MCU until the second valve assemblycontrolling flow from the supplemental fuel source is adjusted to reducethe opening to provide a near stoichiometric mixture flowing from theMCU to the mixing chamber.

Source gas is introduced to the MCU by opening the source shutoff valve.Introduction of source gas into the MCU will typically result in anrichened mixture as sensed by the oxygen sensor resulting in a commandfrom the microprocessor to the dithering valve for increased vacuum. Thevacuum applied to the upper chamber acts to pull the diaphragm, andconsequently the plunger, upwardly opposing the bonnet spring. As theplunger moves up, the supplemental fuel valve is closed and mixturecontrol by the MCU is achieved by motion of the plunger responsive tothe vacuum applied to the diaphragm allowing the first valve controllingthe source gas entering the MCU to open and close proportionate to themovement of the plunger.

As previously described, if the heating value of the source gas beingprovided decreases, the engine begins to run lean and the exhaust gasoxygen concentration increases. This leaner mixture sensed by themicroprocessor results in a command to the dithering valve to decreasevacuum applied to the diaphragm causing the plunger to move downwardlyuntil the valve for the source gas is completely open and the plungerbegins to actuate the second valve adding supplemental fuel to themixture.

Automatic control of engine speed is accomplished using a governorcontrolling the butterfly valve of the gaseous carburetor as previouslydescribed. In the embodiment shown, the governor measures engine speedthrough a magnetic pick up attached to the fly wheel of the engine.Alternatively, sensing of ignition or spark control may be used fordetermining the engine speed. If engine speed decreases, the governoropens the butterfly valve slightly admitting more outside air to theengine. If engine speed increases, the governor closes the butterflyvalve admitting less outside air. The engine governor may be a directspeed sensor control to the butterfly valve or may be routed through themicroprocessor by providing the speed signal to the microprocessor whichin turn provides a voltage control for positioning of the butterflyvalve by a servo motor. As previously described since the source gasoxygen level may rise as a result of entrained air in the source, speedcontrol of the engine through the butterfly valve is essential.

With the source gas shutoff valve indicator switch engaged indicatingthe source gas shutoff valve fully open, initiation of supplemental fuelflow through the second valve in the MCU will result in a flowindication through flow meter 28 of FIG. 1. To increase the flow ofsource gas to the mixing chamber with the first valve of the MCUentirely open, the microprocessor commands the bypass valve 56 to openallowing source gas to flow directly through fuel line 58 to the mixingchamber. As previously described, engine speed control is accomplishedthrough both the butterfly valve and controllable bypass valve asentrained oxygen level in the source gas increases. In a condition ofdepletion of the heating value of the source gas to a level wherein thebypass valve is completely open and controlling engine speed, furthermixture richening is achieved by continued opening of the second valvein the MCU allowing supplemental fuel to flow to the engine. In theembodiment shown, a selected duty cycle for the dithering valve providesan indication of maximum desirable supplemental fuel flow indicatingeffectively complete depletion of the hydrocarbons in the source gas. Inthe present embodiment, a 60% duty cycle has been chosen to providesufficient operating band width for the dithering valve through themixture range desired. When a 60% duty cycle is reached on the ditheringvalve under command of the microprocessor, the system is in a conditionfor shutdown.

FIGS. 3a, 3b, and 3c demonstrate a present embodiment for the mixingchamber. The mixing chamber provides for mixing of outside air receivedthrough the carburetor, source gas with entrained air in some quantityreceived through the bypass valve, and a control fuel mixture comprisingsource gas and supplemental fuel as required received from the MCU. Themixing chamber provides a flange 310 for attachment to the intakemanifold of the engine. As best seen in FIGS. 3a and 3b, two apertures,312A and 312B, are provided for interfacing with the intake manifold.Aperture size, number and arrangement will depend on the internalcombustion engine employed in the system. A top flange 314 as best seenin FIG. 3b is provided with mounting bolts 316 for attachment of thegaseous carburetor. A single aperture 318 is provided in the top platefor fuel and air flow from the carburetor to the mixing chamber. As bestseen in FIGS. 3b and 3c, the back plate of 320 of the mixing chamber isprovided with attachment bolts 322 for fitting attachment from thebypass fuel line 58. An aperture 324 is provided in the back plate forflow of the bypass source gas into the mixing chamber. A side port 326is provided to receive the fuel mixture from the MCU.

Internal baffles divide the mixing chamber into a fuel receiving volume,a swirl chamber and a manifold supply plenum. The fuel receiving volume328 is formed by a bottom baffle 320 welded to the back plate below thebypass source gas entrance aperture, a vertical baffle 332 extendingupwardly from the bottom baffle, and a top baffle 334 extendingforwardly from the vertical baffle. Each of these baffles is sealed tothe right and left side walls.

The swirl chamber is formed by an upper intermediate baffle 336sealingly attached to the vertical baffle and sealed to the right sidewall 342 and front wall 344, a center baffle 338 sealingly attached tothe right side wall, front wall and left side wall 346, and a lowerintermediate baffle 340 sealingly attached to the front wall, left sidewall and vertical baffle. The baffle arrangement as described provides aclockwise rotating series of rectangular apertures formed by thebaffles, forcing air entering through the top aperture of the mixingchamber to swirl and mix with fuel comprising supplemental fuel receivedfrom the carburetor with the primary air, mixture control fuel, andsource gas with entrained air received in the receiving volume of themixing chamber. A plenum 348 beneath the bottom baffle receives themixed fuel air charge to be drawn into the intake manifold throughapertures 312A and 312B. In the embodiment shown, a rod 350interconnects the baffles for structural strength and for manufacturingconsiderations. In the embodiment shown, sealing of the baffles isaccomplished by welding.

The mixing chamber shown in the drawings is a rectangular weldment. Acasting or other forming method with modification of the shape orplanform may be used to create the mixing chamber elements.

Having now described the invention in detail as required by the patentstatutes, those skilled in the art will have no difficulty in modifyingthe elements of the invention to meet specific needs. Such modificationsare within the intent and scope of the invention as defined in thefollowing claims.

What is claimed is:
 1. A mixture control unit to vary the amount andcombination of a source gas, and a supplemental fuel to provide amixture control fuel to an internal combustion engine, the mixture unitcomprising:a body having an internal chamber and further having anoutlet aperture from the chamber, a first valve for introducing sourcegas into the chamber, the first valve having a range of motion fromfully closed to fully open, a second valve for introducing supplementalfuel into the chamber, the second valve having a range of motion fromfully closed to fully open, a bonnet attached to the body forming asecond chamber between the body and bonnet, a diaphragm bisecting thesecond chamber to form an upper chamber and a lower chamber, thediaphragm having a range of motion from a first position through anintermediate position to a second position responsive to a vacuum levelapplied to the vacuum port, means connecting the diaphragm to the firstcontrollable valve, the connection means positioning the firstcontrollable valve from the fully closed position to the fully openposition responsive to position of the diaphragm between the firstposition and intermediate position, the connection means connecting thediaphragm to the second controllable valve to position the secondcontrollable valve from the fully closed position to the fully openposition proportional to the position of the diaphragm between theintermediate position, and second position, resilient means urging thediaphragm into the second position.
 2. A mixture control unit for usewith an internal combustion engine connected to vaporize and burn wastehydrocarbons from a source gas, the mixture control unit receiving thesource gas and a supplemental fuel for combination as a mixture controlfuel, the mixture control unit comprising:a body having an internalchamber, first aperture through the body into the chamber, a first valveseat surrounding the first aperture, a first valve cap adapted to engagethe first valve seat, a first arm pivotally attaching the valve cap to afirst pin whereby the valve cap may rotate from a first fully closedposition engaging the first seat to a second fully open position, afirst lever extending from the first arm opposite the pivot pin, a firstspring engaging the lever urging rotation about the pivot pin to thefully closed position, a second aperture through the body into thechamber for introduction of the supplemental fuel, a second valve seatsurrounding the second aperture, a second valve cap adapted to engagethe second valve seat, a second arm connected to the second valve cap toallow pivotal rotation about a second pin to rotate the second cap froma fully closed position engaging the second valve seat to a fully openposition, a second lever extending from the second arm opposite thesecond pivot pin, a second spring engaging the second lever to urge thesecond valve cap to the fully closed position, a bonnet enclosing avacuum chamber, a diaphragm bisecting the vacuum chamber into an upperand lower chamber, the diaphragm having a range of motion from a firstposition through an intermediate position to a second position, a vacuumport in the bonnet connecting the upper chamber to a controllable vacuumsource, a third spring urging the diaphragm to the second position, aplunger connected to the diaphragm, the plunger engaging the first leveragainst the urging of the first spring to rotate the lever about thefirst pivot pin through the range of motion of the diaphragm from thefirst position to the intermediate position, the plunger engaging thesecond lever to rotate the second lever about the second pivot pinagainst the urging of the second spring through the range of motion ofthe diaphragm from the intermediate position to the second position.